The present invention relates to a process for the manufacture of discontinuous fibrils by the abrupt release of pressure on a two-phase liquid mixture of molten polymer and solvent which is under elevated pressure and at elevated temperature.
It is well known that one can produce continuous fibrillated structures or rovings by similar processes. For example, in U.S. Pat. No. 2,372,695, issued May 15, 1940, and assigned to Celanese Corp. of America there is a description of the production of a downy mass formed of very fine filaments connected with one another by bringing about an abrupt pressure release through an appropriate orifice of a solution of a cellulose derivative which is at elevated temperature and pressure.
According to Belgian Pat. No. 568,524 of June 11, 1958, in the name of E. I. Du Pont de Nemours, continuous structures consisting of a multitude of fibrillous strands or sections, which come together and separate at random intervals to form a "unit fibrillous plexus", are obtained by extruding a solution of synthetic polymer, which is at a temperature higher than the normal boiling point of the solvent and under autogenous pressure or under a higher pressure, through an orifice of suitable shape into a zone of lower pressure.
The fibrillated structures obtained according to the processes described above take the form of continuous rovings. Moreover, as is stated in Belgian Pat. No. 568,524, these structures are produced at a very high speed (which may reach as much as 13,700 m/min), which makes it impossible to cut them up by mechanical means.
The subsequent processing of these continuous rovings produced at very high speed is very difficult. Moreover, for a large number of applications, it is essential to use the fibrillated products in a shreaded form, that is to say in the form of discontinuous structures of relatively short length, for example of the order of a few millimeters. That is why, as can be seen from French Pat. No. 1,246,379 of Nov. 17, 1959, in the name of E. I. du Pont de Nemours, it is necessary to reduce the length of the continuous fibrillated rovings by a treatment in a grinding apparatus. This treatment is harmful to the physical qualities of fibrillated structures and calls for a supplementary operation which necessitates tying up large amounts of capital and requires a considerable amount of power.
It is apparent that a process of the type described above, but leading to the direct acquisition of short fibrils could in numerous cases permit a more economical and easier use of the products obtained and also improve their quality.